Achensee, Alpine lake in Tyrol, Austria

Achensee is the largest lake in Tyrol, stretching between the Karwendel range to the west and the Brandenberg Alps to the east. It reaches a maximum depth of 133 metres, and its water is so clear that visibility extends up to ten metres below the surface. As an alpine lake, its temperature rarely exceeds 20 °C, even in summer.

Regular boat services began in 1887 with the launch of the steamer St. Josef, opening new connections between the villages along the shore. Since 1927 the water has been used to generate electricity, supplying power to the surrounding region.

The lake is nicknamed the "Tyrolean Sea" and the "Fjord of the Alps" for its size and the winds that suit sailing and windsurfing. Its shores and hinterland, especially Pertisau, form the setting of the first thirteen Chalet School books by Elinor Brent-Dyer, who had visited the Tyrol in the 1920s.

Shore villages are Pertisau, Maurach and Buchau in the south, plus Achenkirch with Scholastika and Achenseehof in the north. The west shore between Pertisau and Scholastika is reachable only on foot: an asphalted path to the Prälatenbuche viewpoint, then the narrow Mariensteig with steps to the Gaisalm hut, which has its own boat landing. The northern Gaisalmsteig is secured in places with wire ropes and demands sure footing and a head for heights.

Underwater visibility often reaches ten meters, drawing divers to explore the depths. For nearly a century part of the water has flowed through a power station, generating electricity for the area.